Hydrogen is a colorless, odorless gas used in petroleum reforming, semiconductor manufacturing, cryogenic cooling, chemical synthesis, fuel cells, rocket engines, hydrogen storage, fuel cells, automobiles, fire warning systems, leakage detection, nuclear reactors, sensing environmental contamination, and in biomedical procedures. Hydrogen has also been recognized as an alternative, clean, and renewable energy source. Hydrogen can penetrate into metals and affect their strength and durability. There are also risks of explosion in systems employing hydrogen. For these and other reasons, hydrogen sensors such as electrochemical sensors, metal-oxide resistive sensors, optical fiber sensors, and mass spectrometric sensors have been developed over the years that can monitor the concentration of hydrogen.
Electrochemical hydrogen sensors use a liquid electrolyte and a gas permeable membrane for hydrogen to reach the electrolyte. These sensors can operate from 0.02% to 100% by volume. Exposure of the membrane to cryogenic or time-varying temperatures can affect gas diffusion and make the sensor unreliable.
Metal oxide-resistive sensors rely on a change in electrical conductivity due to an interaction between surface species such as oxide, protons, and hydroxide and hydrogen. In many cases, these types of sensors need to operate at elevated temperatures for effective detection of hydrogen.
Optical fiber sensors utilize the absorption change of an evanescent field in the clad region near the surface of the core of the fiber. In most of these sensors, a thin palladium or palladium alloy layer is usually employed as the transducer, because palladium allows the selective detection of hydrogen. The refractive index of a thin palladium layer changes when it is exposed to hydrogen. By monitoring the optical power transmission of a Pd/Pt-coated optical fiber, one can detect the refractive index changes in the Pd layer, and from this, the hydrogen concentration. This type of sensor has limited sensitivity and reliability.
Mass spectrometers are sensitive and have good linearity over a wide dynamic range. However, the complexity and high cost of mass spectrometers requires skilled operators and special sampling systems for effective hydrogen monitoring. Memory effect can also be a problem in mass spectrometric technology because a vacuum system is used for detection.